Oxygen levels in flight are broadly the same s on the ground, about 210,000 parts per million by volume - hat is about 21%.

However, at cruise altitude the cabin pressure is lower than on the ground at around 82kPa, equivalent to about 1,800m (6,000 ft).

For comparison, air pressure at sea level is 101kPa. At this low pressure, oxygen levels in the blood are lower than at sea level. A healthy person suffers no effects, but those with respiratory illness sometimes need additional oxygen.